This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-284074, filed on Sep. 18, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser for use as a light source in optical communication and optical data recording.
2. Description of the Related Art
At present, semiconductor lasers are widely used as light sources in optical communication, optical data recording and similar fields. This is because they emit a coherent light beam, operate as high speed and can be made small. The semiconductor laser is sensitive to the ambient temperature. Its optical output well changes with the ambient temperature. To control the output of the semiconductor laser with high precision, the output light of the laser is monitored and subjected to feedback control in a drive current circuit. This method is called xe2x80x9cautomatic power control (APC).xe2x80x9d
The output light of a semiconductor laser of edge emission type can be monitored, no matter whether it has been emitted from the front end or the rear end. Nonetheless, front APC is desirable, in which part of the light emitted from the front end of the laser is monitored.
Jpn. Pat. Appln. KOKAI Publication No. 4-232185 discloses an example of front APC using a semiconductor layer. In the APC disclosed in the publication, a light-sensitive element monitors that part of a beam that has not passed the effective area of the focusing lens, not a data-writing beam that has been emitted from a semiconductor laser and passed, in its entirety, through the effective area of the focusing lens. Thus, the APC can be performed without wasting any part of the data-writing beam. In the APC system disclosed in the publication, the semiconductor laser, the focusing lens, and the light-sensitive element are arranged and positioned independently of each other. Here arises a problem. Each component of the APC system must be positioned with high precision. This increases the manufacturing cost of the APC system.
Jpn. Pat. Appln. KOKAI Publication No. 2001-15849 discloses an APC system that solves this problem. This APC system comprises a mount substrate that is composed of an n-type Si substrate, an i-type Si layer provided on the n-type substrate, and a p-type Si layer provided on the i-type Si layer, each epitaxially grown. The substrate, i-type layer and p-type layer constitute a pin photodiode. The mount substrate has a recess made in one part. A semiconductor laser is mounted on the bottom of the recess. The recess has a sloping side. A half mirror covers a part of the sloping side. The half mirror is either a multi-layered dielectric film or a thin metal film. The half mirror reflects a part of the light emitted from the semiconductor laser. The light thus reflected travels upwards from the substrate. The other part of the light passes through the half mirror and is absorbed in the depletion layer that has developed in the i-type Si layer. The light absorbed changes to a photocurrent. The photocurrent is supplied to an APC circuit. In accordance with the photocurrent, the APC circuit controls the output of the semiconductor laser.
If the light emitted from the semiconductor laser is applied to an optical disk, it will be reflected and will travel back toward the semiconductor laser. Hereinafter, the light reflected and traveling back will be referred to as xe2x80x9coptical feedbackxe2x80x9d. In the APC system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-15849, a greater part of the optical feedback reflected from a half mirror, and the remaining part of the optical feedback reaches the light-receiving section of the photodiode, i.e., light monitoring means. More precisely, a part of the optical feedback is absorbed in the depletion layer existing near the i-type Si layer. The optical feedback absorbed in the depletion layer changes to a photocurrent. The photocurrent is added to the photocurrent generated from the light emitted from the semiconductor laser. The sum of these currents is input to the APC circuit. The optical feedback absorbed in the depletion layer results in a noise in the APC circuit. The noise makes a problem particularly when the APC circuit operates at high speed.
As indicated above, the conventional semiconductor laser device comprises mount substrate, a semiconductor laser and a light-sensitive element, both mounted on the same mount substrate. A part of the laser beam emitted from the semiconductor laser is applied to the light-emitting element via the half mirror. In this configuration, the optical feedback reflected from an optical disk or the like travels through the half mirror, inevitably reaching the light-sensitive element. This optical feedback becomes a noise in the APC circuit.
An object of the present invention is to provide a semiconductor laser device mounted on the mount substrate, and a light-sensitive element also mounted on the mount substrate, and in which the light reflected from an optical disk or the like is prevented to reach the light-sensitive element, thereby greatly reducing noise during the use of an APC circuit.
According to the first aspect of the invention, there is provided A semiconductor laser device comprising: a semiconductor laser element, having a light-emitting surface, configured to emit a light beam from the light-emitting surface, the light beam including a central part and an another part; a mount structure configured to mount the semiconductor laser element, including a semiconductor substrate having a surface, and a semiconductor layer having a sloping surface opposed to the light-emitting surface and deposited on a part of the surface of the semiconductor substrate to form a light-sensitive element having a light-receiving section on a part of the sloping surface, the light-sensitive element configured to receive the another part of the light beam from the light-emitting surface; and a total reflection mirror provided on the sloping surface of the semiconductor layer and extending in a direction perpendicular to the direction in which the sloping surface is inclined, the total reflecting mirror configured to reflect the central part of the light beam and outputting the central part of the light beam outside the semiconductor laser device.
According to the second aspect of the invention, there is provided a mount structure comprising a semiconductor substrate having a recess to form a light-sensitive element, the recess provided in a part of the semiconductor substrate and having at least one sloping surface, and the light-sensitive element located outside the recess and having a light-receiving section at a part of the sloping surface; a semiconductor laser element mounted on a bottom surface of the recess and having a light-emitting surface which opposes the sloping surface; and a total reflection mirror provided on a part of the sloping surface, extending in a direction perpendicular to a direction in which the sloping surface is inclined and arranged to reflect and output a part of a light beam emitted from the semiconductor laser element, including at least a central part of the light beam, and to allow the remaining part of the light beam to reach the light-receiving section of the light-sensitive element.
According to the third aspect of this invention, there is provided a mount structure comprising a Si substrate having a main surface and doped with impurities of a first conductivity type in high concentration, a first Si layer provided on the main surface of the Si substrate and doped with impurities in low concentration and a second Si layer provided on the first Si layer and doped with impurities of a second conductivity type in high concentration, a sloping surface being formed at a side part of each of the first and second Si layers, the Si substrate having a trench on the side of the main surface, the trench having a surface that continues to the sloping surface; the Si substrate, the first Si layer and the second Si layer constituting a pin photodiode having a light-receiving surface located in the sloping surfaces; a semiconductor laser element mounted on the mount structure and having a light-emitting surface which opposes the sloping surfaces; and a total reflection mirror provided on the sloping surface and arranged to reflect and output a part of a light beam emitted from the semiconductor laser element, including at least a central part of the light beam, and to allow the remaining part of the light beam to reach the first Si layer, and having a slit-shaped opening made in the sloping surface and extending in a direction perpendicular to a direction in which the sloping surface is inclined.